Since your cabinet is white, I'd try to get a cabinet touch up kit at your local hardware store. Home depot has them in a few colors. If nothing else, it will fill in the low spot and create a flat surface for you to repaint the entire door.

Also, you may need to get a paint that is meant for cabinetry and trim. Usually it is a urethane based paint. Should be able to be color matched if you bring the door into the shop.